1. Field of the Invention
Aspects of the present invention relate to a cap assembly to prevent leakage of an electrolyte and to improve welding, and a secondary battery using the cap assembly.
2. Description of the Related Art
A secondary battery, such as a lithium secondary battery, is a chemical battery that can be repetitively charged and discharged. As portable electronic devices, such as portable phones, notebook computers, PDAs, etc., have been widely distributed, the demand for secondary batteries has greatly increased worldwide.
Lithium secondary batteries include lithium metal secondary batteries, lithium ion secondary batteries, lithium ion polymer secondary batteries, lithium polymer secondary batteries, and so on. Lithium ion secondary batteries have been widely used in electronic devices, such as portable phones, camcorders, digital cameras, notebook computers, and MD players, because they have a high operating voltage (3.6 volt), high performance, and are light-weight.
A lithium ion secondary battery comprises: an electrode assembly including a positive electrode plate, a negative electrode plate, and a separator; and electrolyte to transfer lithium ions. A lithium ion secondary battery can explode, when a transition metal is exposed to air, due to leakage of the electrolyte, or if the internal pressure is suddenly increased, due to a chemical reaction during over-charging. Accordingly, a protection module should be provided for commercial usage.
A lithium polymer secondary battery has benefits in that there is no risk of explosion, and it can be implemented in a variety of shapes, because it contains a gel-type electrolyte. A lithium polymer secondary battery comprises an electrode assembly including a positive electrode plate, a negative electrode plate, and a separator disposed between the positive electrode plate and the negative electrode plate, to transfer lithium ions.
Since a lithium polymer secondary battery uses a high molecular weight, gel-type electrolyte, it can have a strong resistance to chemical reactions resulting from over-charging and over-discharging. Accordingly, a lithium polymer secondary battery does not require a protection module.
Lithium ion secondary batteries can be prismatic, cylindrical, and pouch-type, according to a shape of casing containing the electrode assembly. In a prismatic lithium ion secondary battery, the casing can generally be made of a metal, such as aluminum. Further, a lithium ion secondary battery may be prismatic, or cylindrical.
When constructing a secondary battery, an opening is formed at one side of a casing, an electrode assembly is inserted into the casing through the opening, and then an electrolyte is injected. Next, the opening is sealed with a cap assembly. The cap assembly may comprise a cap plate that corresponds to the opening. An electrolyte inlet may be formed on the cap plate, through which an electrolyte is injected.
The secondary battery is charged a first time, and gas generated during the charging is removed. During the gas removal, the electrolyte may leak from the electrolyte inlet, and stain an area around the electrolyte inlet. Accordingly, when an electrolyte inlet cap is welded to the electrolyte inlet, the electrolyte may compromise the weld.